Patients require rehabilitation for walking as a result of aging, stroke, or other injuries. Such rehabilitation is typically performed under the supervision of a medical professional, and frequently in the context of a medical facility, such as an outpatient clinic or other specialized rehabilitation clinic. Rehabilitation includes the use of walkers that help ease the load from the feet, helping the patient to correct an uneven gait or limp, while the patient simultaneously relearns muscle coordination and builds muscle strength. These known apparatuses, including conventional walkers and wheelchairs, are not entirely satisfactory for the range of applications in which they are employed. U.S. Pat. No. 9,278,042 discloses a walker that is vertically and horizontally adjustable. The walker has four caster wheels which each pivot 360 degrees. The entire walker pivots easily and requires that a user remain confined within the walker on all four sides of the walker. Although the confinement provides safety for the user it makes the walker impractical for one who wishes to use a vertically and horizontally adjustable walker outside the confines of a clinic or hospital. What is needed is a vertically and horizontally adjustable walker that is open in the rear and facilitates entering and leaving the walker from the rear of the walker, while remaining stable with regard to rotation of the walker.